In recent years, measures to environmental issues are requested worldwide. Especially, an energy saving request has become severe to home electronics products such as a washing machine and an air conditioner. A microcomputer installed with a PWM output apparatus is widely used to control a 3-phase motor installed in each of the home electronics products.
Generally, the outputs of the 3 phases of U phase, V phase, and W phase are used to control the 3-phase motor. The PWM output apparatus outputs PWM control signals of a positive phase and a negative phase for each phase to control the 3-phase motor. In order to rotate the 3-phase motor precisely, the ON/OFF period of each of the PWM output signals of the positive phase and the negative phase used to control the 3-phase motor must be adjusted finely. However, it is very difficult to carry out the precise adjustment due to a wiring delay to the motor and temperature variability. Therefore, it is desired that the realization of the PWM output apparatus possible to control in well precision and finely.
A positive phase PWM output signal and a negative phase PWM output signal are complementary to each other, and basically, when one of them is in an ON condition, the other is in an OFF condition. However, there is a possibility that the positive phase PWM output signal and the negative phase PWM output signal are turned on simultaneously due to a wiring delay from the microcomputer to the motor unit and temperature characteristics. When the positive phase PWM output signal and the negative phase PWM output signal are turned on simultaneously, a short-circuit current flows through a motor control circuit. It is desired to avoid such a phenomenon in order to guarantee a normal operation of the motor unit.
A method of setting a dead time is known to prevent that the positive phase PWM output signal and the negative phase PWM output signal are turned on simultaneously. The dead time is a period which is set for the motor control to prevent that the positive phase PWM output signal and the negative phase PWM output signal are turned on simultaneously. During this period, the positive phase PWM output signal and the negative phase PWM output signal are both set to an inactive condition. The setting of the dead time is called a dead time insertion.
A technical example in which the dead time is inserted is disclosed in Non-Patent Literature 1. A microcomputer in Non-Patent Literature 1 is provided with a PWM output signal block and a dead time inserting block. This example has a purpose of controlling the 3-phase motor by adjusting the PWM output width in units of clock cycles, while avoiding that the positive phase PWM output signal and the negative phase PWM output signal are turned on simultaneously.
FIG. 1A and FIG. 1B are block diagrams showing a circuit configuration of a saw-teeth waveform (triangular waveform) PWM output apparatus of Non-Patent Literature 1. This circuit is provided with a PWM generator 300, a PWM output width setting register 301, a PWM output signal 302 outputted from the PWM generator 300, a dead time inserting block 303, a dead time setting register 304, and a PWM output signal 305 after the dead time insertion. The PWM output signal 302 has six signal lines. The signals of the positive phase and the negative phase of each of the U phase, the V phase, and the W phase of the 3-phase motor are transmitted by these signal lines.
The dead time inserting block 303 inserts a dead time into the PWM output signal. Specifically, after a negative phase PWM output signal is set to the inactive condition, a positive phase PWM output signal is set to an inactive condition for a dead time period. Moreover, after the positive phase PWM output signal is set to the inactive condition, the negative phase PWM output signal is set to the inactive condition for the set dead time period. By such an operation, even when a difference occurs between the positive phase and negative phase PWM output signals, it can be avoided that the positive phase and negative phase PWM output signals become active simultaneously.
An example of the PWM output signal 302 outputted from the PWM generator 300 of the above PWM output apparatus is shown in FIG. 2A and FIG. 2B. The PWM output width of 4 clock cycles is set to the PWM output width setting register 301 and the dead time of 2 clock cycles is set to the dead time setting register 304.
The positive phase and negative phase PWM output signals of the PWM output signal 302 before the dead time insertion are respectively shown as the PWM output signal (positive phase) 401 and the PWM output signal (negative phase) 402. In this condition, a simultaneous ON condition does not occur. However, in an actual device, due to influence of a wiring capacity and so on, a delay sometimes occurs in a rising edge and/or falling edge of each of the positive phase and negative phase PWM output signals. In order to avoid the simultaneous ON condition even in such a case, the dead time is inserted.
When a delay occurs at the switching of the PWM output signal (negative phase) 402 from a high level to a low level, the dead time is inserted into the PWM output signal (positive phase) 401. The PWM output signal (positive phase) 403 shows a waveform after the dead time period 405 of 2 clock cycles is inserted. The timing when the PWM output signal (positive phase) 403 is switched from the low level to the high level is delayed by the 2 clock cycles, compared with the PWM output signal (positive phase) 401 outputted from the PWM generator 300.
In the same way, when a delay occurs at the switching of the PWM output signal 401 (positive phase) from the high level to the low level, the dead time is inserted in the PWM output signal (negative phase) 402. The PWM output signal (negative phase) 404 shows a waveform after the dead time period 406 of the 2 clock cycles is inserted. The PWM output signal (negative phase) 404 is delayed by the 2 clock cycles in the switching timing from the low level to the high level, compared with the PWM output signal (negative phase) 402 outputted from the PWM generator 300.
In the above-mentioned related technique, the simultaneous ON condition of the positive phase PWM signal and the negative phase PWM signal can be prevented by inserting a constant dead time period at the rising edge or falling edge of the PWM output signal.